1. Field of the Invention
The present invention relates to an aqueous ink jet recording liquid used in an ink jet recording method and an ink jet recording method using the same.
2. Description of the Related Art
Printers utilizing an ink jet recording method in which recording is effected on paper, cloth, film and the like by discharging liquid or melted solid ink through nozzles, slits or a porous film or the like have various merits such as compact size, cheapness, silence and the like, and are widely commercially available as black mono-chromatic or full-color printers. Among others, a so-called piezo ink jet method using a piezoelectric element, or a so-called thermal ink jet method in which recording is conducted by forming liquid drops by the action of thermal energy has a lot of merits such as being able to obtain high speed printing and high resolution, and the like.
Conventionally, an water-soluble dye-based ink jet ink generally used in ink jet recording methods has excellent long-term storage stability, but has problems in water resistance and lightfastness. On the other hand, an ink using a pigment as a coloring material has excellent water resistance and lightfastness, and can provide images having high density without bleeding; therefore, such ink is extremely promising, and recently, a lot of proposals regarding the ink have been tried out, or put to actual use. For example, Japanese Patent Application Laid-Open (JP-A) No. 56-47871 proposes a recording liquid composed of an aqueous medium containing at least a pigment, a polymeric dispersant and a non-ionic surfactant. U.S. Pat. Nos. 5,085,698 and 5,221,334 propose an AB or BAB block copolymer as a dispersant for a pigment. U.S. Pat No. 5,172,133 proposes using a specific pigment, a water-soluble resin and a solvent.
The pigment-based ink jet inks has the merits described above, at the same time, they had problems in reliability and image-fixing property. One reason for this was that since the pigment is insoluble in the aqueous medium and is dispersed in the aqueous medium, the pigment flocculates. This flocculation of the pigment is believed to occur in cases in which the ink is stored for a long period of time or stored at high temperatures, in cases in which water and the like evaporate and the pigment becomes concentrated at a distal point of a nozzle or relative concentrations of other ink additives increase, or in cases in which a dispersion state of the ink changes due to shock caused by applications of energy pulses for jetting the ink. Such flocculation of the pigment would cause problems in ink jet recording described below, and also is a cause of shortened printing head life.
(1) Physical properties of the ink change, and printing performances varies with this change.
(2) Accumulation of the pigment at an ink flow route or at the tip of the nozzle causes clogging of the nozzle, leading to poor discharging.
(3) Due to pollution of the ink flow route with the pigment, bubbles tend to remain in the ink flow route, and decreased ink jet property and poor discharging occur.
(4) When the ink flow route is polluted with the pigment, particularly in the thermal ink jet method, accumulation of the pigment on a heater can be a cause of burning on the heater (kogation).
Therefore, as methods for suppressing flocculation, methods described below which aim towards improving stability of a dispersed system are disclosed.
Japanese Patent Application Laid-Open (JP-A) No. 5-331391 and the like describe a method in which flocculation of the pigment is suppressed by electrostatic repulsion between molecules, by restricting the amount of a specific ionic impurity to a fixed amount or less, since ionic impurities are known to weaken the electrostatic repulsion between pigment surfaces. Japanese Patent Application Laid-Open (JP-A) Nos. 1-204979 and 6-1936 describe a method in which the particle size of the pigment in the ink is restricted to a fixed level or less, utilizing the fact that when the particle size is smaller, the dispersed system is more stable. Further, as a general theory, a method which utilizes a steric effect of a polymeric dispersant is also known.
In using an ink jet recording apparatus, it is expected that water and a water-soluble organic solvent and the like evaporate at the distal point of the nozzle, and that an ink composition changes greatly from the initial condition. However, there has been made no dispersion stabilization design in which even in such a concentrated condition, the coloring material does not flocculate, and the ink does not undergo excessive thickening and solidification.
The present invention provides an aqueous ink jet recording liquid which has excellent stability over long periods of time, and in printing, causes no clogging of an ink flow route, prevents kogation, provides a recorded material having excellent rub resistance, printing quality, optical density, water resistance and lightfastness, and can prevent uneven density, due to being able to preserve dispersion stability of a coloring material even in regards to deterioration with age of an ink composition.
The present inventors and the like as a result of earnest studies, have concluded it is essential that the ink jet recording liquid satisfies the following conditions in order to satisfy dispersion stability and printing quality simultaneously.
Namely, the present invention is an aqueous ink jet recording liquid comprising at least water, a water-soluble organic solvent and a water-insoluble coloring material, wherein
(1) the absolute value of the zeta potential is 20 mV or more,
(2) the conductivity is from 0.05 to 0.75 S/m,
(3) the number average particle size of dispersed particles is from 15 to 200 nm, and
(4) the number of the particles having a particle size of 0.5 xcexcm or more present in one liter is 2.5xc3x971011 or less.
Further, the present invention is preferably an aqueous ink jet recording liquid comprising at least water, a water-soluble organic solvent and a water-insoluble coloring material, wherein
(1) the absolute value of the zeta potential is 20 mV or more,
(2) the conductivity is from 0.10 to 0.50 S/m,
(3) the concentration of an alkaline metal ion is from 500 to 3000 ppm,
(4) the number average particle size of dispersed particles is from 20 to 80 nm,
(5) the volume average particle size of the dispersed particles is from 30 to 250 nm, and
(6) the number of the particles having a particle size of 0.5 xcexcm or more present in one liter is 2.5xc3x971011 or less.
In the present invention, the above-described conclusions have been reached based on the following considerations.
Namely, in an environment in which ordinary ink jet recording apparatuses are used, since water and a water-soluble organic solvent and the like evaporate from an ink discharging portion, a composition, structure and physical properties of an ink are liable to change, and failures such as nozzle clogging and the like are liable to occur, particularly in the vicinity of the ink discharging portion.
To pre vent nozzle clogging, it is possible to consider a method in which the ink is designed so that due to the evaporation of water and the like, the water-insoluble coloring material flocculates, forming a suitable structure, excessive evaporation of water and the like is prevented by this flocculated structure, and printing is made possible by breaking down the flocculated pigment at a tip end of a nozzle by maintenance recovery operations (dummy jet, vacuum suction, wiping and the like) attached to a recording apparatus when carrying out printing, and the like.
In the present invention, it has been decided to carry out an ink design in which the ink composition changes greatly from the initial state, the coloring material dose not flocculate even in a concentrated state, and the ink dose not thicken or solidify. For this purpose, singly controlling the particle size or the conductivity of the water-insoluble coloring material or the like is not effective, and it has been concluded that it is necessary to control the free charge state of the surface of the water-insoluble coloring material in the ink and dispersion stability against in regards to deterioration with age in the condition of the ink.
On the other hand, the ink jet recording liquid is considered to exhibit behavior as described below on a recording material.
(1) Stability of the water-insoluble coloring material decreases and flocculation occurs due to evaporation of water and the water-insoluble organic solvent.
(2) The water-insoluble coloring material permeates the recording material, and the water-insoluble coloring material is fixed onto the recording material.
(3) Electrostatic interaction operates between the recording material and the water-insoluble coloring material, and the water-insoluble coloring material is fixed onto the recording material.
It is considered that when flocculation of the coloring material on the recording material is quick, high optical density and prevention of image bleeding can be attained, but rub resistance of image becomes worse. On the other hand, it is considered that when the water-insoluble coloring material maintains a finely dispersed state even on the recording material, although rub resistance of image is excellent, image bleeding and optical density are poor. Therefore, to prevent image bleeding, impart high optical density and attain excellent fixing property, it is believed to be necessary that the water-insoluble coloring material takes on a suitable flocculated structure on the recording material.
Occurrence of uneven density when printing a pigment ink is closely related to dispersion stability of the ink. Namely, coarse particles remain on paper due to flocculation during storage of the ink and emphasize light and shade to generate xe2x80x9cuneven densityxe2x80x9d.
As a result of these considerations, it has been concluded that the ink jet recording liquid of the present invention comprising at least water, the water-soluble organic solvent and the water-insoluble coloring material can satisfy dispersion stability and printing quality simultaneously by satisfying specific conditions regarding the zeta potential, the conductivity, the number average particle size of the dispersed particles, and the number of the particles having a particle size of 0.5 xcexcm or more present in one liter.
The aqueous ink jet recording liquid of the present invention exhibits excellent characteristics in long-term storage stability, in heat/cold cycles or in allowing the recording liquid to stand in a state in which the print head is filled with the recording liquid and in the like, is excellent in regards to printing characteristics such as nozzle clogging and kogation, and is also excellent in regards to printing qualities such as rub resistance, water resistance, lightfastness, image bleeding, optical density, uneven density and the like.